Pain in the cervical, dorsal, and lumbar vertebrae often originates from posterior vertebral joints or zygapophysory joints or “facet” joints. When medical therapy is no longer sufficient for easing the pain, in particular when such joints have become very deformed by arthrosis or old trauma, it becomes desirable to replace their joint surfaces by prostheses; the purpose of such prostheses is to conserve the functions and mobility of the vertebral column, and to avoid the joints becoming blocked (arthrodesis).
Documents FR 2 721 501 and U.S. Pat. No. 6,132,464 describe various devices for replacing the joint surfaces of posterior vertebral facets; each of those devices comprises a support having a convex face fitting closely and coming into bearing contact against a portion of the concave surface of the posterior arc of a vertebra, on at least one side of the spinous process; the support is fixed to the vertebra by a pedicular screw, and/or to the spinous and transverse processes by encircling means such as a band, a collar, or a hook; the device further comprises a strip forming half of a joint prosthesis and having two opposite faces: a first face known as a “joint” or as a “sliding” face which is in contact with a similar face of an adjacent vertebra with minimum friction; said first face being covered in a biocompatible material that slides well, such as stainless steel or titanium, in particular; and a second face opposite from the first face of the strip, known as the “bone” face, which is porous and covered in spikes, is optionally covered in hydroxyapatite, and is intended to bear against the remaining bone of the joint process; the strip is connected to the support by a sessile base whose convex and porous posterior face bears against the bone of the vertebral arch; the means for fixing the support to the spinous and transverse processes and to the pedicle are associated with one another as a function of the anatomic conditions encountered so as to ensure stability for the assembly.
Those devices present certain drawbacks: because of the presence of numerous respective bearing faces on the various portions (support, strip, base) of the device on different regions of a vertebra, it is difficult to ensure that bearing takes place with quality that is uniform and satisfactory because of the variations in shape between one vertebra and another and/or one patient and another; in addition, the presence of the base, the anchoring support, and of the various fixing means contributes to increasing the volume of the prosthesis and consequently to increasing the discomfort for the patient and the trauma due to implanting such a device.